1. Field of the Invention
The present invention relates to a method for measuring the speed of a vehicle travelling on a track of railway type.
The present invention also relates to the installation for carrying out this method.
2. Description of the Related Art
Various systems for determining the speed of a train travelling on a track have already been proposed. In particular, it has been suggested to use a sensor present on an axle to determine the speed of the train travelling on the track. However, this speed is not always sufficiently precise, and in particular, it might not take account of a risk arising when the wheel skids for reasons such as the climatic conditions (frost or snow) or the presence of leaves on the rails.
It has also been proposed to place two or three sensors on different axles in order to obtain better precision. However, this remains insufficient from the point of view of the risk management.
It is also known practice to arrange beacons along railway tracks in order to measure the speed of the vehicle travelling on these tracks. In this case, beacons, which are arranged at known and fixed distances, emit a signal. The vehicle travelling close to this beacon detects, with the aid of an antenna, the passage over the first beacon and measures the time upto the passage of the second beacon. The speed is readily deduced from the known distance between the two beacons and the time taken by the vehicle to cover this distance. Nevertheless, the beacons are placed a relatively large distance apart and this amounts essentially to measuring the average speeds over the distance covered.
It has also been proposed in document WO97/12796 to use a calibrated beacon to determine the almost instantaneous speed of a vehicle passing in its vicinity. This beacon emits a magnetic field and, by means of an antenna placed under the vehicle, this vehicle can detect the entry into and exit from this field of magnetic influence. The time taken by the vehicle to cross the field of magnetic influence is deduced therefrom, and the speed of the vehicle is thus calculated. This method has the drawback of needing to place beacons at regular distances along the tracks.
Moreover, it is known practice to organize a track into track sections known as “block-sections”, which are separated by electric joints. An electric joint consists of two tuning blocks acting as the power coupling for the track sections adjacent to each tuning block and for the short length of track located between these two tuning blocks (15 to 30 meters). Usually, the first tuning block acts as an emitter at a given frequency while the second tuning block acts as a receiver at another frequency. The functions of the electric joint are, firstly, to prevent the propagation of the signal from one track circuit to the adjacent track circuit and, secondly, to couple the emitter and the receiver with the track.
It is already known practice to use an electric joint to detect the passage of a train. Actually, on passage of the train axles, a short-circuit is created between the two rails via the train axles and thus enables the detection of the position of said train relative to the emitter from the change of current in the track. Specifically, it is observed that the current at the F1 frequency in the rail in front of the axle is high before the axle passes at the level of the emitter connection, and undergoes a strong discontinuity at the moment the axle passes.
The document GB-A-2 153 571 describes an example of a track circuit assembly that is particularly suitable for a short track circuit of less than 40 m in length, which may be used in underground railway transit systems.
It is mentioned therein that an electrical short-circuit is produced between the rails and that an AC signal control unit is connected approximately 6 meters later so as to tune the loop thus formed to the resonance, to the frequency of the selected track signal. The control units comprise a capacitor, the value of which is chosen so as to adjust the resonance, and a transformer, one coil of which is mounted in series with the capacitor, a track circuit signal emitter or receiver being connected via a second coil of the transformer.